LM48411

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SNAS399G – SEPTEMBER 2007 – REVISED MAY 2013

DC input coupling which eliminates the two external AC coupling, DC blocking capacitors. The LM48411 can be

used, however, as a single ended input amplifier while still retaining it's fully differential benefits. In fact,

completely unrelated signals may be placed on the input pins. The LM48411 simply amplifies the difference

between the signals. A major benefit of a differential amplifier is the improved common mode rejection ratio

(CMRR) over single input amplifiers. The common-mode rejection characteristic of the differential amplifier

reduces sensitivity to ground offset related noise injection, especially important in high noise applications.

PCB LAYOUT CONSIDERATIONS

As output power increases, interconnect resistance (PCB traces and wires) between the amplifier, load and

power supply create a voltage drop. The voltage loss on the traces between the LM48411 and the load results is

lower output power and decreased efficiency. Higher trace resistance between the supply and the LM48411 has

the same effect as a poorly regulated supply, increased ripple on the supply line also reducing the peak output

power. The effects of residual trace resistance increases as output current increases due to higher output power,

decreased load impedance or both. To maintain the highest output voltage swing and corresponding peak output

power, the PCB traces that connect the output pins to the load and the supply pins to the power supply should

be as wide as possible to minimize trace resistance.

The use of power and ground planes will give the best THD+N performance. While reducing trace resistance, the

use of power planes also creates parasite capacitors that help to filter the power supply line.

The inductive nature of the transducer load can also result in overshoot on one or both edges, clamped by the

parasitic diodes to GND and VDD in each case. From an EMI standpoint, this is an aggressive waveform that can

radiate or conduct to other components in the system and cause interference. It is essential to keep the power

and output traces short and well shielded if possible. Use of ground planes, beads, and micro-strip layout

techniques are all useful in preventing unwanted interference.

As the distance from the LM48411 and the speaker increase, the amount of EMI radiation will increase since the

output wires or traces acting as antenna become more efficient with length. What is acceptable EMI is highly

application specific. Ferrite chip inductors placed close to the LM48411 may be needed to reduce EMI radiation.

The value of the ferrite chip is very application specific.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM48411 contains shutdown circuitry that reduces

current draw to less than 0.01µA. The trigger point for shutdown is shown as a typical value in the Electrical

Characteristics Tables and in the Shutdown Hysteresis Voltage graphs found in the Typical Performance

Characteristics section. It is best to switch between ground and supply for minimum current usage while in the

shutdown state. While the LM48411 may be disabled with shutdown voltages in between ground and supply, the

idle current will be greater than the typical 0.01µA value.

The LM48411 has an internal resistor connected between GND and Shutdown pins. The purpose of this resistor

is to eliminate any unwanted state changes when the Shutdown pin is floating. The LM48411 will enter the

shutdown state when the Shutdown pin is left floating or if not floating, when the shutdown voltage has crossed

the threshold. To minimize the supply current while in the shutdown state, the Shutdown pin should be driven to

GND or left floating. If the Shutdown pin is not driven to GND, the amount of additional resistor current due to the

internal shutdown resistor can be found by Equation 1 below.

(VSD - GND) / 300kΩ

(1)

With only a 0.5V difference, an additional 1.7µA of current will be drawn while in the shutdown state.

AUDIO AMPLIFIER POWER SUPPLY BYPASSING FILTERING

Proper power supply bypassing is critical for low noise performance and high PSRR. Place the supply bypass

capacitor as close to the device as possible. Typical applications employ a voltage regulator with 10µF and 0.1µF

bypass capacitors that increase supply stability. These capacitors do not eliminate the need for bypassing of the

LM48411 supply pins. A 1µF capacitor is recommended.

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